Talk:Black hole/Archive 4

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Why is the Universe here?

Let me break in here to point out that this talk page is not intended to be a place where people can ask informational questions; the top of this page now contains a pointer to some places you can try instead! ---CH 04:58, 23 May 2006 (UTC)

As far as I can make out the universe began as a singularity. Now surely this would mean that it would be classed as a black hole, so how did it expand like it did? Presmably its swarzchild radius would be larger than it's mass radius?

You wrote now surely this would mean that it would be classed as a black hole, but that doesn't follow. The defining feature of a black hole is a global property, the event horizon, not a local property, such as the existence of a curvature singularity. (Unfortunately, this distinction is probably too technical for me to attempt to explain in a few nontechnical words here!) There are many spacetime models in gtr which exhibit curvature singularities but no event horizons, such as most cosmological models, various pp-wave spacetimes, etc.

BTW, the term you want is Schwarzschild radius. Parse that schwarz (black) schild (shield), which may help you to remember the correct spelling. See Karl Schwarzschild. ---CH 00:21, 16 March 2006 (UTC)

Information and Black holes

Let me break in here to point out that this talk page is not intended to be a place where people can ask informational questions; the top of this page now contains a pointer to some places you can try instead! ---CH 04:58, 23 May 2006 (UTC)

Couldn't information on what had gone into a black hole be derived from the rest of the universe. I.e., until an object crosses the event horizon, it will be interacting with the rest of the universe? So assuming you knew enough about the rest of the universe, couldn't you deduce what had entered a black hole from information that had never entered the black hole (i.e. motions imparted on other objects by forces from that object (gravity, electromagnetism, collisions, etc.)? The object itself would be irretrevable, but wouldn't the legacy it left with the rest of the universe be plain to see? My second question is as to why the assumption is made that the Laws of Physics are the same in either direction. Doesn't the 2nd law of thermodynamics violate this? linguofreak

From a quantum mechanical point of view, it's fairly easy to end up with information contained in the object that can't be derived from its history or inferred from its effect on the rest of the universe until the object is explicitly probed for it. I'm pretty sure that you can conduct classical scenarios where this applies too, but would have to doublecheck with the lurking physicists to be sure. You can easily construct scenarios where the information is stored in a classical manner, but can't be observed or inferred due to the fact that the particles carrying information are exchanged as discrete quanta, but this is a different type of situation.

As for the apparent conflict between thermodynamics and time symmetry, it turns out that this isn't actually a conflict. The second law of thermodynamics describes the behavior of systems of particles - the way the average statistics of the system will evolve over time. The laws of physics govern local interactions between particles, not aggregate behavior explicitly, and are approximately time-symmetrical (though not quite; they're invariant under CPT symmetry, not T-symmetry).

To show how this works, consider a situation where you had several coins on a table, and flipped a random coin to a random state once a second. The behavior of any given coin is the same whether you look at things forward or backward in time: the coin's state changes to a random new state at random intervals averaging once per N seconds. However, if I set all of the coins initially to heads (a state with low entropy), I observe the system as a whole to evolve in a time-dependent manner (going forwards in time, the entropy increases, and I get a system with a mixture of heads and tails). Running the film backwards shows the opposite (the mixture sorts itself out into an all-heads configuration). Only when entropy is near its maximum does the arrow of time disappear (when the coins are a roughly even mixture of heads and tails, running the film in either direction shows it random-walking around this region of state space). This is discussed a bit at T-symmetry.

So, you can have a system with time-symmetrical rules, whose global behavior is not symmetrical if it starts from a state with low entropy. The universe appears to be a system of this type. --Christopher Thomas 06:52, 29 March 2006 (UTC)

Also, why is it seen to be neccesary that information be preserved? I.e, why shouldn't a black hole eat information? Linguofreak 23:38, 2 April 2006 (UTC)

Originally it was thought that they did "eat information"; this is discussed at black hole information paradox. It seemed odd that information would be destroyed in this manner, as information appears to be conserved in most other processes that we know about (it's intimately tied to entropy and thermodynamics in ways that I'm sketchy about). The short version is that the entropy of a system can be thought of as being its information content. This made black hole evaporation via Hawking radiation seem to be a violation of the laws of thermodynamics. --Christopher Thomas 02:21, 3 April 2006 (UTC)

404s in References

We're going to need to cite alternate references, many of the ones in this article are permanently gone. As and when I find them, I'll edit them in, but that's relying on my memory. We also have two References sections. Or did have. I'm renaming the former to "Citations", since that's what it is. Wayne Hardman 21:51, 29 March 2006 (UTC)

Contribution by 208.230.68.137

Research done by Tiffany Marie for my powerpoint presentaion

A black hole is the escape speed that would have to be met to escape from the pull of gravity exerted upon an object. The existence of black holes in the universe is well supported by astronomical observation, particularly from studying X-ray emission from X-ray binaries and active galactic nuclei. The gravitational field is so strong that the escape velocity past its event horizon exceeds the speed of light. This implies that nothing, not even light, inside the event horizon can escape its gravity. It is, however, theorized that wormholes can let one exit a black hole. In 1784, John Michell realized that gravity could attract light and form "dark stars". Michell ideas were published by Pierre Simon Laplace in first editions of an astronomy guide. In 1795, Laplace defined the requirements for a black hole. Einstein's general theory of relativity describes gravity as a curvature of spacetime caused by the presence of matter. If the curvature is fairly weak, Newton's laws of gravity can explain most of what is observed. For example, the regular motions of the planets. Very massive or dense objects generate much stronger gravity. The most compact objects imaginable are predicted by General Relativity to have such strong gravity that nothing, not even light, can escape their grip. The Hubble Space Telescope (HST) is a telescope in orbit around the Earth. Its position outside the Earth‘s atmosphere allows it to take sharp optical images of very faint objects, and since its launch in 1990, it has become one of the most important telescopes in the history of astronomy. It has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics. Hubble's Ultra Deep Field is the deepest (most sensitive) astronomical optical image ever taken. The Chandra X-ray Observatory is part of NASA’s fleet of “Great Observatories” along with the Hubble Space Telescope, the Spitzer Space Telescope Telescope and the now deorbited Compton Gamma Ray Observatory. Chandra allows scientists from around the world to obtain unprecedented X-ray images of exotic environments to help understand the structure and evolution of the universe. Already surpassing its five-year life, NASA's Chandra X-ray Observatory is rewriting textbooks and helping advance technology.

—Preceding unsigned comment added by 208.230.68.137 (talk • contribs) (this machine is apparently located in Washington, PA and registered to Armstrong Cable Services, which is headquartered in Butler, PA)

Tiffany, I don't know who you presented this "presentation" to, but your very first sentence is completely wrong. (You defined, not quite correctly, "escape velocity", not "black hole".) Fortunately the rest is mostly not so wrong, just very disorganized. So were you trying to get feedback, or what? ---CH 04:24, 22 April 2006 (UTC)

What are the particles of Hawking Radiation?

Let me break in here to point out that this talk page is not intended to be a place where people can ask informational questions; the top of this page now contains a pointer to some places you can try instead! ---CH 04:58, 23 May 2006 (UTC)

This is a general question about virtual particle pairs.

In a note at the foot of this page the particles are described as electrons and positrons. But, is it only electrons that appear or do quantum processes allow for any particle-antiparticle pairs to be created, e.g. protons, neutrons, photons, etc?

Also, at what rate are these created? Are we talking about 1 an-hour, or 1,000,000 a-second?

Finally, what is their density? 1 per square metre (per hour), or 1,000,00 per square millimetre (per second)?

Many thanks!

Sorry I can't give you any figures, but keep in mind that protons and neutrons are much heavier than electrons, therefore it takes a lot more energy to create one (E=mc²). So, protons and neutrons are much rarer, but some of them are still produced, as are any kind of particle.

Also note that the rate at which a black hole evaporates depends on its size. Small black holes evaporate faster, and they eventually disappear in a puff of radiation. —Keenan Pepper 13:40, 4 April 2006 (UTC)

My understanding is that photons were the most common thing produced, but that all particle/antiparticle pairs with masses that are lower than the energy corresponding to the hole's temperature are produced. --Christopher Thomas 19:28, 4 April 2006 (UTC)

Kinda. The probability of making a pair decreases exponentially with its rest energy. (But don't quote me on that. =P) —Keenan Pepper 00:04, 5 April 2006 (UTC)

Mass of the earth?

"Following the explosion, only the core of that star will remain which, if it it is massive enough (about three or four times that of the earth) its own gravity causes the core to implode and suck everything around it into this newly formed black hole."

Surely the Sun? (Ajkgordon)

Sun, yes. This is the TOV limit. --Christopher Thomas 19:27, 4 April 2006 (UTC)

Hang on, Ajkgordon, suck everything around it into this newly formed black hole is potentially misleading (no Newtonian limit puns, please!) since a black hole won't tug any harder than any other object with the same mass, it is just more compact, which means you can get close enough to experience (in a stellar mass hole) some rather large tidal forces. ---CH 04:08, 22 April 2006 (UTC)

CH notes an important point. A more accurate way of describing things would be to say that in the collapse of a star sufficiently massive, the compacted core of neutron-degenerate matter that forms during collapse quickly becomes heavier than the TOV limit, at which point it collapses, and all of the infalling matter in the collapsing star falls into the newly-forming black hole (as there is no surface left to impede its collapse). --Christopher Thomas 04:30, 24 April 2006 (UTC)

Black holes and SF plot device

Let me break in here to point out that this talk page is not intended to be a place where people can ask informational questions; the top of this page now contains a pointer to some places you can try instead! ---CH 04:58, 23 May 2006 (UTC)

Assume using the usual science fiction technobabble that a miniscule black hole has been created/placed at the centre of an Earth size planet, with a sun at the same point in its life cycle as ours.

How long before the inhabitants of the planet/observation base notice that "summat odd" was happening to the planet, and what would they observe? Transmission of seismographic waves/earthquake activity across the diameter of the planet would be affected (as minding the gap) - but when? And at what point would seismic activity change (ie volcanoes etc) because the circulation of material within the lower layers has ceased?

Jackiespeel 20:57, 7 April 2006 (UTC)

If the black hole was just put there, they'd notice when the planet disappeared from under them. How long this takes depends on how massive the hole is. I'd have to handwave interaction cross-section numbers to see how long it would take a hole with a radius smaller than an atom to eat the planet. Anything with the mass of a large asteroid or higher would eat it very quickly.

If a rigid shell was built under the Earth's crust, and everything inside replaced with a hole of equal mass, we'd notice immediately from the way seismic waves propagated. We'd notice within months that continental drift had stopped (if the shell was right under the crust), or longer that there was a change in flow patterns if the shell was somewhere in or under the mantle. How long it would take for the heat within the crust and mantle to dissipate (and stop volcanic activity) depends on where the shell is built. Either way, it would be very obvious very quickly that the shell was present (no more seismic waves passing through the core, or mantle, or what-have-you). A shell like this would be many orders of magnitude stronger than any presently-known material, including nanotubes.

Even if a shell was built, you'd need active station-keeping mechanisms to hold the hole in place. Otherwise any small perturbation off-centre would wind up being amplified, and the hole would drift ever more quickly off towards the crust. When it got close, it would probably eventually eat most of the rest of the planet (while inside Earth's roughly spherical shell, it wouldn't feel much gravity from Earth's mass, but outside it would. The bits of crust would all be in orbit around each other and the hole, and the end result would probably be a hole with most of Earth's mass surrounded by a debris ring. How long it would take the hole to drift off-centre depends on a number of parameters and assumptions, so I'm not even going to try to hand-wave it for now.

It's an interesting thought-experiment, though. --Christopher Thomas 21:13, 7 April 2006 (UTC)

Watching a History of the Earth category program - and there was a dire SF short story I read a long time ago - the proverbial ancient civilisation on Mars using a very tiny black hole to power their McGuffin and the humans exploring managed to let it escape and I put the ideas together. I would be satisfied only a rough timescale - eg ten years, within the natural lifespan of the Solar System, longer.

I presume an asteroid sized black hole would consume the sun somewhat slower than it would the Earth (g).

The plot device (if anyone cared to develop it as a story) would need sufficiently plausible technobabble but not technodump.

Jackiespeel 21:43, 8 April 2006 (UTC)

By "asteroid sized," do you mean mass or radius? An asteroid-radiused black hole should eat the sun very quickly. Linguofreak 22:22, 8 April 2006 (UTC)

The short story would be "The Hole Man", by Larry Niven. He wrote another, later story called "The Borderland of Sol" that featured a micro black hole under other conditions. A small black hole would actually probably eat the sun more quickly than it would eat Earth, as the sun's core is much denser. The early stages of absorption are the ones that most affect the total time, and the denser the hole's environment, the more interactions per second, and the more quickly it grows. As for time frame, this is extremely sensitive to the size of the hole. It would span all the way from "happens in milliseconds" to "takes time comparable to the age of the universe". The long-term limit depends on the lower bound to the mass of the hole, as limited by Hawking radiation. The smallest hole of interest would still have to have a lifetime of billions of years. Anything smaller would be shining brightly enough to interfere with matter around it and muck up calculations further. The important thing is that a black hole's interaction cross-section, and so the rate at which its mass increases, goes up as the square of its mass (or faster, once you take "gravitational focusing" of trajectories into account). This gives exponential growth, meaning that almost all of the hole's devouring happens at the very end of the time it takes to eat something. The approximation breaks down when the hole is big enough that a) bulk material can simply fall into it, and b) its mass becomes comparable to or greater than that of the object it's eating. As a rule of thumb, you can assume that the final collapse of stars, planets, and so forth happens at the speed at which their surface material would freely fall into a point object with the star or planet's mass. While the material is still far from the hole, this won't be blindingly fast (one gravity for Earth, hundreds to thousands for a middle-sized star). --Christopher Thomas 16:45, 10 April 2006 (UTC)

I was quoting the previous mention of asteroids. Would the dropped black hole go to the middle of the planet or through it (China syndrome)? As the host body rotates, would the black hole do so - and once the body begins to be hollowed out would a gyroscope effect emerge?

Would a neutron star be swallowed up more quickly than the star in its previous form?

I think my question is being answered (and I hope others find it an interesting topic).

Jackiespeel 21:23, 10 April 2006 (UTC)

The micro-hole would pass through Earth as if it wasn't there, as it is vastly more massive than the amount of matter in the channel it's carving. It would behave as if in orbit around Earth's centre of mass, though the amount of matter influencing it varies (as Earth is roughly spherical, to the first order it acts like only the matter beneath it, not above it, exists). If it was created in a lab on Earth's surface, it would follow a not-really-elliptical path that had it matched with Earth's rotation at its highest point. If it impacted from space, it would pass through Earth, be deflected a bit, and go right back out into space again (capture would actually be pretty difficult in this case). If it orbits inside Earth for a while, it'll eventually absorb enough mass (over seconds-to-billions-of-years) to have slowed down relative to Earth, and orbit inside it until the final collapse. Regarding neutron stars, they would indeed collapse extremely quickly, as they are extremely dense (the hole would encounter a lot of matter that it could absorb very quickly, and the final collapse would proceed at accelerations of hundreds of thousands to millions of gravities). --Christopher Thomas 22:23, 10 April 2006 (UTC)

non-active black holes

Let me break in here to point out that this talk page is not intended as a place where you can ask informational questions about black holes; see the header above for some suggestions about places where you can ask such questions. ---CH 04:59, 23 May 2006 (UTC)

I read that the black hole at the center of our galaxy is no longer active. What does that mean? The Secretary of Funk 02:37, 10 April 2006 (UTC)

IIRC, it means that it is no longer pulling in large amounts of matter and spewing it out in jets. I've paid more attention to Linguistics than Astronomy over the past couple years though, and my beloved old Astronomy textbook is in pieces. "No longer active" doesn't say so much about the hole itself as its surroundings: there's no longer anything in the immediate vicinity that hasn't either gone into a stable orbit or been sucked in. Linguofreak 04:11, 10 April 2006 (UTC)

Yes: it is now thought that all or almost all galaxies harbour supermassive black holes; long ago, these powered what we now see (in optical images of very far and very long ago galaxies) as quasars, in which case astronomers have long spoken of active galactic nuclei (AGNs). Hence, we have the picture of galactic nuclei starting off very active and gradually settling down into a kind of inactive senility. ---CH 04:12, 22 April 2006 (UTC)

Quantum Singularity

Can someone help figure out what to do with the article for Quantum Singularity? It's a mess, so I was hoping that someone here could either help at least get it started or lend an opinion as to whether it is deletable? Thanks. Madmaxmarchhare 02:47, 11 April 2006 (UTC)

It looks like it should be drastically compacted and merged into a "list of black holes in fiction" list somewhere. I can't find any suitable merge target offhand, though. If you want to put it on AfD, go ahead, as it would likely pass. --Christopher Thomas 04:07, 11 April 2006 (UTC)

Would it be right to just re-direct Quantum Singularity to Black Hole (disregarding what's in there already). Sorry, physics was a long time ago for me... Madmaxmarchhare 17:14, 11 April 2006 (UTC)

There's enough activity on the page that it should probably go through a proper AfD process. See WP:AFD for directions on how to do this. For an article like this, it's a formality, but going through the process demonstrates your good intentions. --Christopher Thomas 20:41, 11 April 2006 (UTC)

My gut feeling, actually, is that "quantum singularity" is a common enough term in fiction that it deserves its own article. I cleaned up the article and added an intro that relates the term to real physics as far as is warranted, and moved it to Quantum singularity (fiction). But if you still think it ought to be deleted, I won't object—Wikipedia:Proposed deletion may be the way to go, since it happens automatically after five days (without a vote) if there are no objections. -- SCZenz 22:30, 11 April 2006 (UTC)

I generally agree that it probably deserves its own article since it appears so often in science fiction, etc. I think what you did will work as a good start and nicely blends nominal theory with the reality of its use in fiction. At this point, I'm apt to let it stand and gather. Madmaxmarchhare 23:46, 11 April 2006 (UTC)

3tries

i earned a c, i'm amaerican, and i can spell, in physics, and i wanna apply something.  

ok:: eventually someone might wanna figure our bigbang spot as 0,0,0. we are so obviously multidimensional that the straight, flat line, has overshadowed not solely in its dominance, yet also because of its simplicity. ok:: mapping our visible galactic neighbors in space we can set forth parameters of distance(time is still relative and unimportant) so that triangulations can be determined allowing a realistic picture of where we live. in time and repetition more and more of our universal world will become visible, telling us how big our space is. black holes are fun because they might be the keylock pieces to our space if they are massable eventhough the notion of superdense mass, mounting, and maybe stationary in our collective movement, is something. so... a better, truer? understanding of the relation, the relativity of our space as we get to know it might allow some visions of the shape of those areas of black hole meaning traceable wormholes- that last was a bit far however, the blackhole mass could be the structure of a tube if multiple outlets could be found. maybe superdense singularities exist

—Preceding unsigned comment added by Bigrobhollins (talk • contribs)

You're "amaerican" and you can spell? Anyway, you can't just ignore time and try to find "our bigbang spot as 0,0,0" because space and time are mixed up with each other. That's basic special relativity. —Keenan Pepper 07:34, 16 April 2006 (UTC)

In the spirit of Wikipedia, I suppose we should not discourage new editors, but at the same time, it seems fair to ask that newbies avoid creating an account and posting their debut WP edit while (apparently) in a state of extreme intoxication. Since I'm picturing Rob as a underaged drinker, one might even add a plea to avoid extreme intoxication, period, but I guess that's your business, Rob. Or was "Rob"'s incoherent comment someone's idea of a "joke"? ---CH 03:04, 22 April 2006 (UTC)

The additions by 81.102.97.175

from Luton, England (IP registered to NTL Internet, UK) have already been reverted by Christopher Thomas, and a good thing too, but assuming good faith, I'll offer some pointers:

1. Although most scientists are not entirly sure what happens within a "black hole": true, and the classic paper by Israel on black hole interiors is a good starting point; see also colliding plane wave models and their applications to this question,
2. it is widely thought that the Laws of physics do not apply: false, of course physicists expect that some theory is capable of accurately modeling black hole interiors, and indeed most physicists expect that general relativity should be perfectly capable of modeling the region not to far inside the horizon (a fringe minority dispute this, as do many cranks); however, it is widely expected that gtr should break down near putative curvature singularities in the "deep interior", where curvatures approach L^-2 where L is the Planck length,
3. to something with such a great mass: false; it is not the mass but the compactness of an object which determines whether or not it forms a black hole,
4. and therefore time could be slowed down to a standstill: false, you are no doubt thinking of what happens just outside the event horizon of a Schwarzschild hole, but even there you are confused: of course time does not "slow to standstill" anywhere in any spacetime; rather, radio signals from a static test particle suspended (say on a long and very strong but neglible mass string) just outside the event horizon will be extremely redshifted when received by static observers far from the hole; this does not imply that time has "slowed" for the suspended particle, just that outgoing null geodesics diverge due to the spacetime curvature outside the hole (or indeed any massive object),
5. or even not exist at all: half true; it seems likely that spacetime must be replaced by some underlying concept near curvature singularities,
6. this means that theoretically matter could be compressed to an infinate singularity: half true; in general, gtr fails to model what happens to matter which encounters a curvature singularity, but not all singularities which (according to gtr) might be found deep inside black holes are "crushing"; the ones inside the simplest gtr models of black holes are, but the Poisson-Israel model includes the possibility that a particle falling into the interior will encounter a Cauchy horizon rather than a crushing singularity.

In the spirit of Wikipedia we should try to avoid discouraging new editors, but it seems fair to ask that newbies be very sure of their facts (and be able to cite standard textbooks or highly reputable printed math/science encyclopedias to back up their claims!) before editing articles on scientific topics. To soften the blow, I confess that I and our Luton anon do share one characteristic: we would both benefit from a spell-checking utility!---CH 02:53, 22 April 2006 (UTC)

The "photo" of a black hole

The manipulated picture that supposedly represents the appearance of a black hole... seems to me perhaps to be incorrect? Maybe I'm just stupid, but I couldn't find discussion on this picture in the discussion page, so I'm raising it here. It's just that I intuitively imagine a black hole as appearing... sort of the opposite to how it appears in that picture - in the picture it looks like light is being bent away from the black hole instead of toward it, and the images are pushed outwards, if you will, instead of sucked inward. Does anyone understand what I'm saying, and can anyone confirm or deny this picture's accuracy? Thanks. - 61.9.204.168, 22nd April, 2006 (Big Pond, Telstra TPP, Australia)

I'm assuming you mean Image:Black_Hole_Milkyway.jpg, which shows a black hole against a star field background. While I initially had some of the same concerns you did, after looking at it carefully it seems the picture is correct. What's happening is two things. First, intuition ends up giving the wrong answer for what gravitational lensing looks like. Consider a light path drawn from us that passes out into space far from the hole. It isn't deflected significantly, and what we see on that path is the same as what we'd see if the hole wasn't there. As we move the path towards the hole, though, the hole bends the path. A change in our viewing angle by some amount X results in an outbound ray that's deflected by more than X. This means that in a relatively small region of our view, we see more of the background than we'd expect to -- in other words, the background is compressed radially, which is exactly what's shown in the picture.

The second thing that's happening is that the simulated picture is from a viewpoint very close to the hole. As light paths get very close to the hole, they aren't just deflected, but can wrap around the hole and be sent back to us (or wrap around farther if the hole is spinning, but the hole in the picture isn't). The pink ring in the image is the angle from the hole at which light paths from us are focused on a single point in the background (i.e., are deflected enough to make them perpendicular to the page, heading into the page). Inside the ring, paths are warped even more, giving a reversed image of the entire starfield, including parts that don't show in the periphery of the picture.

Lastly, paths that pass close enough to the hole are warped in a way that sends them into the horizon. This makes the hole's horizon appear larger than we would measure it to be by feeding a cable with measuring devices down towards it. A black hole of 10 solar masses would have a diameter of about 60 km, taking up an angle of 0.1 radian at a range of 600 km in the absence of gravitational lensing. The image width is about 1.6 radians (pi/2, or 90 degrees), but the hole in the image looks a lot larger than 1/16 of the image width.

I hope this description is useful to you. --Christopher Thomas 07:04, 22 April 2006 (UTC)

Just to amplify: if our Big Pond anon has seen a discussion of the lensing equation, that belongs to weak-field lensing theory and is appropriate for astronomical observations of distant galaxies lensed by another object at intermediate but large distance. The image in question (from Physics, University of Tübingen) depicts a view from a static observer close to a non-rotating (Schwarzschild) hole and uses strong-field lensing theory. In this theory, we take account of the fact that very near the horizon, null geodesics coming in from "infinity" can wrap around arbitrarily often before escaping again to "infinity". (Some null geodesics, of course, plunge into the hole, but many of those which graze the limb of the sphere $r=3*m$, very roughly speaking, do manage to escape.) The result is that a static observer will in principle see multiple images of the entire night sky in concentric annuli around a dark disk. The dark disk is not the event horizon. Roughly speaking, it can can be thought of as the image of the photon sphere. In weak-field theory, we do not attempt to treat geodesics which come so close to the hole that they orbit the hole several times before escaping. See for example Chandrasekhar, Mathematical Theory of Black Holes.

However, there are a range of things which one should take into account in order to compute an image which accurately models what one would see. AFAIK, most computed images neglect at least some of these; for example, the image you were complaining about may neglect intensity in order to focus on computing just the relativistic ray-tracing. It is also worth pointing out that the "view" seen by a nonstatic observer can be radically different from the view seen by a static observer. See for example Andrew Hamilton's visualization. Also, the view near a spinning hole is more complicated. And if your hole is surrounded by an accretion disk, the view becomes even more interesting (and more complicated to model). See also the recent reminiscence by Brandon Carter, which conveniently has a relevant appendix and also reprints a famous image computed by Luminet several decades ago.---CH 00:13, 25 April 2006 (UTC)

Question

Let me break in here to point out that this talk page is not intended to be a place where people can ask informational questions; the top of this page now contains a pointer to some places you can try instead! ---CH 04:58, 23 May 2006 (UTC)

If a black hole is near another black hole, what happens? —The preceding unsigned comment was added by 68.237.238.121 (talk • contribs) on 01:17, 1 May 2006.

I'm glad you'd like to learn more, Anonymous, but I'll suggest you look at the Wikipedia:Reference Desk for a place to put these kinds of questions. This page is for discussion of this particular article.
As for your question, I think the same thing that happens when any other objects get near each other... They will orbit each other, and one may fall into the other, adding its mass to it. Black holes aren't really all that special, it's just that they're very compact... Basically, they do the same thing as two suns when they get near each other, only they pull much more strongly when they get particularly close. *shrugs* - JC 02:58, 1 May 2006 (UTC)

There are some very interesting, and rather unique, phenomena that arise when black holes collide. They are expected, for example, to produce gravitational waves. -- SCZenz 03:13, 1 May 2006 (UTC)

Recent anon edits to this article

Let's briefly see what they've been doing here recently:

1. 24.21.172.229 (talk · contribs), aka the or.comcast.net anon, possibly near Frisco, TX: several silly vandalisms
2. 70.36.48.136 (talk · contribs) reverted one of the latter (USEFUL)
3. 71.254.177.170 (talk · contribs) aka the fios.verizon.net anon, possibly near Murrieta, CA: rude vandalism
4. 204.193.117.66 (talk · contribs) corrected spelling of a word (USEFUL)
5. 68.216.160.66 (talk · contribs) improved section heading (USEFUL)
6. 66.149.51.243 (talk · contribs) aka the biz.mindspring.com anon, possibly in the Northeastern US: altered word to be misspelled
7. 167.128.58.201 (talk · contribs) aka the k12.or.us anon in Corvallis, OR: silly sandbox vandalism
8. 24.171.158.218 (talk · contribs) apparently trying to fix cats (USEFUL)
9. 70.50.229.227 (talk · contribs) apparently trying to improve links (USEFUL)
10. 62.30.16.3 (talk · contribs) corrected punctuation (USEFUL)
11. 86.136.34.122 (talk · contribs) aka the btcentralplus.com anon in London: silly vandalism
12. 206.15.235.3 (talk · contribs) aka the colusa-coe.k12.ca.us anon in La Grande, OR: rude sandbox vandalism
13. 206.78.5.172 (talk · contribs) aka the Visalia Unified School District anon in Visalia, CA: sandbox vandalism
14. 128.232.250.254 (talk · contribs) aka the trinhall.cam.ac.uk anon on the campus of Cambridge University: sandbox vandalism
15. 81.205.106.129 (talk · contribs) aka the adsl-surfen.hetnet.nl anon: silly sandbox vandalism
16. 198.237.17.14 (talk · contribs) aka the Oregon Public Education Network anon, near Roseburg, OR: persistent silly vandalism
17. 66.150.98.217 (talk · contribs) aka the nibble.net anon (Nibble Net is headquartered in Osceola, IN): persistent silly vandalism
18. 207.136.223.5 (talk · contribs) aka the vermont-academy.pvt.k12.vt.us anon at a prep school near Saxtons River, VT: schoolboy vandalism
19. 204.213.179.242 (talk · contribs) aka the ntplx.net anon, possibly in the Northeastern US (Netplex is headquartered in Hartford, CT): massive POV-pushing replacement
20. 68.189.6.27 (talk · contribs) aka the scrm.ca.charter.com anon near Traverse City, MI: deliberately misspelling vandalism

I count twenty anons (note they are listed by domain, not by individual edits). The geolocation refers to the machine, not to the individual using this machine. It is possible that some of these vandals were in fact using several different machines.

Of the twenty possibly distinct anons, six made useful edits and the other fourteen were vandals, often apparently sitting in a junior high school computer lab. One of these was an anon who was clearly trying to push a dissident point of view in violation of WP:NPOV. Two were vandals whose modus operandi consists of altering the spelling of an apparently randomly chosen word from an apparently randomly chosen article so that the word is misspelled. Their only goal is presumably to evade detection. Four vandals were using this article as a sandbox and reverted their own edits.

One of the schoolboy anons (the one from the Vermont prep school) has apparently entered small hoaxes related to his school into various minor articles, again apparently hoping only to evade detection. This anon was briefly blocked by User:Lupo back in February, but has obviously returned to carry out further vandalism.

In or two cases, we can see from the recent history that several edits by different vandals occurred before any correction was attempted. I didn't attempt to check that all vandalism was properly reverted.

On balance, I think this is more evidence that anon edits should be banned, since the harm they do tends to outweight the good some anons do. One thing which becomes clear when you start to look into this is that it only takes a small number of repeat offenders to really mess things up for users of good will. ---CH 03:25, 19 May 2006 (UTC)

This is why Wikipedia:Semiprotection exists. Unfortunately, as I found when I requested it a while back (for nuclear weapon, if memory serves), an article has to be very frequently vandalized for it to be applied (20/day wasn't enough last time). In my opinion the most productive approach to solving the vandalism problem would be to push for a "vandalism edit count vs. non-vandalism edit count" criterion to be included in the semiprotection page making it easier to get it granted. --Christopher Thomas 05:18, 19 May 2006 (UTC)

After 100 anonymous vandalism edits, 100 reversions and 10 useful anonymous edits, the net quality of the article is still higher than it would have been with the blocking of anonymous edits. They are essential to the success of Wikipedia. BigBlueFish 13:31, 29 May 2006 (UTC)

I strongly disagree for two reasons:

• In the last 50 edits, I count the following from non-vandal anons:

• Two typo fixes.
• One link to fringe science.
• One addition of "see also" links, about half of which are relevant.
• One addition of a redundant external link.
• One reversion of vandalism.
• One attempted clarification of an example paragraph that in my opinion makes it less, not more, clear.
• One phrasing tweak to a sentence that made it less accurate ("exit path for light or matter" to "exit path for energy or matter").

While arguably somewhat useful, I hardly consider these "essential" contributions.

• Any anon who is sincere about contributing to Wikipedia would be willing to get an account under which to edit. It doesn't even require an email address nowadays. Thus, even supposing substantial useful edits from anonymous contributors, I don't see how blocking anons would stop those users from contributing.

If all of these anon edits had been blocked, the article would be no worse than it is now, and we'd have had far, far fewer headaches reverting vandalism and a history page that's actually useful to look at. --Christopher Thomas 13:58, 29 May 2006 (UTC)

I agree with Christopher Thomas here. One thing which BigBlueFish might not have considered is that reversions wastes the time of our most knowledgeable contributors, who would prefer to be adding valuable new content rather than reverting silly vandalism of stuff they have already added. This is very much to the detriment of the project, since over time, valuable editors tend to become disgusted and stop contributing. This is Very Bad Thing.---CH 17:16, 29 May 2006 (UTC)

Wikipedia works by letting anons edit, even when it does more harm than good. This article may be hurt by that concept—because it has a very high profile—but if one out of a hundred vandals writes something stupid, realizes that he really can edit a page, and then becomes a contributor, then it was worth it to Wikipedia. There are hundreds of people whose sole contribution to the site is reverting anonymous vandalism, and it is (in their opnion, and mine) worth that effort to keep our contributor base growing. That's why semi-protection won't be approved for this article.

Now, if we're not catching vandals effectively enough for this page, the solution is to watch more carefully, which I will try to do. -- SCZenz 14:16, 29 May 2006 (UTC)

SCZenz, you wrote Wikipedia works by letting anons edit. Many Wikipedians do seem to firmly believe this, and even get very upset when this belief is challenged. Yet the attempts I have seen (one or two of which I have carried out myself) to test whether this is true suggest that it is not. For example, see these stats. In my estimation somewhere between 2/3 and 4/5 of anon edits are bad, and somewhere between 2/3 and 4/5 of all bad edits are made by anons. Such studies directly challenge the claim that overall anon edits are a good thing. I haven't seen any attempt to study directly the claim that substantially fewer individuals would become Wikipedians if anon edits were disallowed, but I doubt this is true. Even if it were true, as some claim, that anon edits were neccessary until WP "took off", I strongly suspect that WP editing is now known to a "critical mass" of the global pool of potential editors to easy/fun, that with their friends and friends of friends "spreading the word", it is unlikely that disallowing anon edits would cut off the influx of "new blood". ---CH 17:12, 29 May 2006 (UTC)

I do not dispute for a minute that most anon edits are bad, or that most bad edits are by anons—this makes sense since users who decide they like editing and want to do so seriously will tend to get accounts, but it doesn't disprove my point in any way. Obviously there's no way to prove I'm right either; I don't think we can tell without an actual experiment. I do know that periodically people who know a lot about physics make valuable edits, and that profs or other high-level experts who don't want to get accounts aren't going to do it just because we disable anonymous editing. I also note that even if there are ten bad anonymous edits for one good anonymous edit, the ten bad edits will be reverted and the one good edit will improve the encyclopedia. -- SCZenz 17:35, 29 May 2006 (UTC)

I still don't think that most of the good edits would be lost by banning anon edits from this article. Anyone who's motivated enough to put in a significant amount of time improving an article will be motivated enough to take the 30 seconds to register a login and password. The people who aren't motivated sufficiently to do this, probably wouldn't put in the effort to make useful contributions either. Also, as CH points out, you have to weigh any good edits that you assume you wouldn't get with anons banned against good edits that you lose from regular editors because we're too busy mopping up dreck to make them. That's one of the reasons I haven't contributed to this article in a while (some of the valid additions need vetting, but I'm not going to crawl through the history to find them all). In short, I find this argument for anonymous edits providing a net gain to this article to be unconvincing. --Christopher Thomas 18:05, 29 May 2006 (UTC)

If you would like to see counterexamples to Hillman's evidence that "anonymous editors usually do not contribute anything substantial" and Thomas' suggestion that "anyone motivated enough to put in a significant amount of time will also register", take a look at the history of algebraic structure. I have no idea if this anon would get an account if that page were sprotected, or what his reasons might be for not registering. He has not responded to several requests on his talk page that he do so. Certainly I will admit that such anonymous editors are the exception, rather than the rule. -lethe ^{talk +} 18:29, 29 May 2006 (UTC)

Here Lethe's tongue that slipped; he did not of course mean counterexample, since no-one (certainly not I) claims that no anon edits are ever good or that no bad edits are by registered users. Lethe means that some anons seem to be doing good work (I agree). I have also pointed out from time to time good anon edits. In fact, this is so rare that when I see them I often take the time to add an "attaboy" in a anon talk page. I have probably spotted 5 good anon edits in random articles during my time at WP. And perhaps 500 bad ones. Unfortunately, the 500 bad ones wind up weighing more that the 5 good ones. That is why we must bad anon edits.---CH 22:14, 29 May 2006 (UTC)

I know people who refuse to get accounts but sometimes make substantial edits. I just don't think it's true that people who edit would automatically get an account if forced. But I don't claim that anon edits are a net gain for this article, just that they're a net gain for Wikipedia. Anyway, this argument is totally academic; the probability that this article will be semi-protected is just about zero. Semi-protection is (by policy) for articles which are unreadable because they're constantly vandalized, not for articles whose regular editors are inconvenienced by having to revert vandalism. -- SCZenz 22:05, 29 May 2006 (UTC)

I've heard that claim before and I dont' doubt you, but it is exasperating that such people never seem to show up to explain in a detailed and thoughtful comment in a discussion like this why they refuse to register an account, perhaps offering the information that the blippetty.bloo.blaa anon in this article should not damn well not exist and it's only a silly example so please don't create it! was their contribution. Another thing: at least I have made some attempt to use statistics to challenge my own impression that Anon Edits are Bad. I admit I don't know quite you could do this, but have you even considered devising some kind of statistical study of your own to challenge your own claim? Which is, as I understand it, that disallowing anon edits would discourage such a large number of anon contributors from continuing to make good edits that their loss would outweigh the benefit gained by eliminating at one stroke the biggest source of bad edits which need to be carefully reverted by the so-called Army of Eyes.---CH 22:22, 29 May 2006 (UTC)

I suppose you're mostly correct about the value of anon contributors, though I think it would be completely impossible to change policy on this. There is an enormous amount of inertia to radical change in policy, and I expect the policy will require a large stimulus to change, say a big lawsuit. Remember how quickly Jimbo enacted his top-down fiat change of anon editing policy during the Siegenthaler controversy? -lethe ^{talk +} 22:37, 29 May 2006 (UTC)

How could I forget? I continue to insist that continuing to allow anon edits will be the downfall of the Wikipedia, and I find it not unlikely that this will come due to the site being raided or the servers confiscated due to gross misuse by anon editors. I'd much rather see the policy changed before that kind of disaster, but only time will tell.---CH 22:59, 29 May 2006 (UTC)

Temperature?

Let me break in here to point at the top of this page, which now contains some links which should persist through archiving, to try to head off questions which are better asked in other forums as suggested above. ---CH 03:40, 23 May 2006 (UTC)

Well, Hello. I must first of all state that I dont know near anything about black holes, All of my knowledge having mostly come from this site and the ocasional highschool Physics Lesson. And it was these lesson wich made me think of these things.

Nº1: Considering the properties of dark elements to absorb all lightwaves and, ergo, appear Dark, taking this in consideration when talking about black holes would it mean that all Lightwaves absorbed by the black hole, wich, literally allowing none to escape it's gravitational force, then Black Holes would have to be Superheated masses, however, considering that Since nothing can escape it's gravitational force:

1-This Enourmous heat would be constricted to the Center of the Black Hole, or considering this jet wich I have seen represented in the site, expelled as a superheated jet behind the Black hole?

2-Would this Heat Exist in the First place?

3-Now that I'v re-read what I'v written, wouldnt there be a possibility for the Photons absorved by the Black hole to be ejected out the extragalactic Jet? Perhaps producing a Bright beam of Light?

Thanks, and, well, pardon my poor English. --201.208.141.154 03:55, 23 May 2006 (UTC)

I moved your comment to the bottom to preserve chronological order. Why do you say black holes would be "superheated"? According to the no hair theorem, black holes are described by only three independent properties: mass, electric charge, and angular momentum. It turns out that it does make sense to assign a "temperature" to a black hole, but that temperature is inversely proportional to its mass, and for normal-sized black holes it's very cold, a tiny fraction of a kelvin. See Black hole thermodynamics for more information. —Keenan Pepper 02:35, 23 May 2006 (UTC)

Also be sure to note that the accretion disk jets are made of material that was never inside the black hole to begin with. Nothing can get out of a black hole, it just acts as a kind of slingshot. —Keenan Pepper 02:40, 23 May 2006 (UTC)

Hmm, thanks for the Info. What I meant when I sayed that It would be superheated was that Given what I proposed the photons would have been transformed to Heat whist being trapped by the mass of the black hole, and since nothing can escape it's gravitational force, then there would'v been lot's of photons getting trapped by the Microsecond and ergo lot's of heat being concentrated aswell. Also, sorry about having placed it on top, I originally meant for it to be placed there to get a fast answer (wich I did), so... Anyways sorry about that.

Also, what I meant was that If that Heat existed it could have been retined in the center of it. But then again, how I sayed, I hardly know anything about this, thanks for the answer.--201.208.141.154 03:55, 23 May 2006 (UTC)

Remember, heat is simply the random microscopic motion of particles. It's impossible to tell whether particles inside a black hole are moving, because you can't see inside a black hole. —Keenan Pepper 03:48, 23 May 2006 (UTC)

Well, thanks for the answers and links. Hope to see you around Wiki. Bye.--201.208.141.154 03:59, 23 May 2006 (UTC)